cpp_double_fp documentation error 2

[jrade]

The same bad header name as in issue #747 can also be found in the table in
https://www.boost.org/doc/libs/latest/libs/multiprecision/doc/html/boost_multiprecision/tut/floats.html

[ckormanyos]

Hi @jrade thanks again for finding this one too. I will repair it also.

[ckormanyos]

See also #750

[jrade]

Also, the backend names in the first column in the table are not correct. The backends are

cpp_bin_float
cpp_dec_float
gmp_float
mpfr_float_backend
float128_backend
cpp_double_fp_backend

[ckormanyos]

Also, the backend names in the first column in the table are not correct. The backends are

Good catch again. It looks like the backend names you suggest correct 3 additional errors.

I will correct accordingly.